Many heavy duty trucks carry a wide range of loads depending on the amount of cargo or materials they may be carrying. To carry the full range of loads, some trucks use one or more non-drive rear axles in conjunction with one or more drive axles. A non-drive rear axle may be needed for a variety of reasons, such as to comply with suspension and tire load ratings, bridge laws, etc. When the truck is unloaded, the non-drive axle is just “along for the ride,” since it is not needed to carry any load in that condition. For this reason, many trucks include non-drive axles that can be lifted off of the ground and into a stored position (the “lifted position” or “raised position”) when not needed. This type of axle is often referred to as a “lift axle,” “liftable axle,” “auxiliary axle” and/or an “auxiliary suspension system.” By raising the lift axle when the vehicle is lightly loaded, wear on the lift axle tires can be reduced and fuel economy can be increased due to the absence of rolling resistance from the lift axle tires.
Lift axles can be installed ahead of or behind the drive axles on a straight truck or tractor. If a lift axle is positioned ahead of the drive axle, the lift axle is referred to as a “pusher.” If the lift axle is positioned behind the drive axle, it is referred to as a “tag.” Lift axles can also be installed ahead of or behind an axle or tandem axles on an articulating trailer. In addition, there are both steerable and non-steerable lift axles. As the name implies, the wheels on a non-steerable lift axle cannot turn left or right, and as a result non-steerable lift axles need to be raised prior to turning a tight corner or the tires will scuff and impose high lateral stresses on the wheels and other chassis components. In contrast, steerable lift axles include wheels that caster so they turn and can remain in contact with the ground during tight turning.
Lift axle systems typically include an axle that is movably coupled to the chassis of the vehicle by one or more control arms on each side of the vehicle. The control arms provide lateral support for the axle, but enable the axle to be raised into the stored position when not in use. When a lift axle is in the lowered or active position, the load on the lift axle (commonly referred to as “sprung mass”) is typically carried by one or more load springs. The load springs are often air springs, which typically include a flexible bellows that forms an air chamber that is inflated to extend the spring and carry the load. Many lift axles also employ additional air springs (commonly referred to as “lift springs” or “lift bags”) that lift the axle into the stored position when the vehicle is in an unloaded or lightly loaded condition. Various lift axle systems and associated components are described in U.S. Pat. No. 5,403,031 to Gottschalk et al., U.S. Pat. No. 6,073,947 to Gottschalk et al., U.S. Pat. No. 6,311,993 to Hulstein et al., U.S. Pat. No. 6,880,839 to Keeler et al., U.S. Pat. No. 9,352,628 to Barton et al., and U.S. Pat. No. 9,688,318 to Wetter et al., each of which is incorporated herein by reference in its entirety.